^ Follicle cells synthesize and secrete thyroglobulin from their apical surfaces into the follicle lumen where it is stored. The follicle lumen is an extracellular compartment and, thus, secretion of thyroglobulin constitutes the exocrine secretion of the follicle cells and accounts for the polarity of the cells. ^ The tyrosines of thyroglobulin are iodinated in the follicle lumen and rearranged to form the thyroid hormones (T3 and T4), which are modified tyrosines that are retained in the primary structure of thyroglobulin.
^ The iodinated thyrogobulin is resorbed by pinocytosis into the follicle cells where it is hydrolyzed, liberating T3 and T4.
^ T3 and T4 are released from the basolateral surfaces of the follicle cell and enter the blood stream. ^ Active and inactive follicles
• Active follicle. Follicle cells are cuboidal to columnar and are involved with both secretion and resorption of thyroglobulin.
• Inactive follicle. Follicle cells are squamous, reflecting the paucity of secretory organelles and the lack of synthetic and uptake activity.
^ The parathyroid glands are four small, spherical glands that are embedded in the posterior surface of the thyroid gland. ^ Cell types
♦ Major cell type, arranged in cords or clumps
♦ Small polyhedron-shaped cells with secretory granules visible only with electron microscope
♦ Secrete parathyroid hormone (PTH) which increases blood calcium levels, primarily by increasing osteoclast activity
♦ Large cell may appear singly or in clumps
♦ Heterochromatic nucleus and abundant eosinophilic cytoplasm, due to numerous mitochondria
♦ No secretory granules
♦ Function is unknown.
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